Automotive heat exchangers, such as air conditioning condensers, fall into three basic configurations or types, tube and fin, serpentine, and parallel flow. All three basic designs are decades old at this point, and each presents unique manufacturing challenges. Parallel flow condensers have a plurality of short flow tubes, running horizontally between long, vertical manifold tanks, with each end of each flow tube joined to a tank in a leak free fashion. Serpentine condensers are unique in not requiring long header or manifold tanks, having only one or two long flow tubes that wend back and forth in a distinctive sinuous pattern from end to end. The obvious drawback is the necessity to create a plurality of U shaped bends in the very long flow tubes. Such integral bends cannot be too sharp, and thereby limit how closely the straight portions of the tubes can be packed and spaced. However, the advantage of not having to assure a plurality of leak free braze joints between multiple flow tube ends and their insertion holes in elongated manifold tanks led to their extensive use, at least before braze technology was improved.
Although tube and fin condensers are generally parallel flow also, in terms of their refrigerant flow pattern, they are generally referred to just as tube and fin condensers, because of the unique, braze free method by which their basic cores are produced. In fact, tube and fin was the first design to be produced in large volumes, because of its relatively low cost manufacturing process. A series of round flow tubes, sometimes straight, and sometimes U shaped, are inserted though holes in planar, flat cooling fins, and expanded out into tight mechanical engagement therewith. Thus, the basic core has the advantage of a braze and weld free conductive connection between the flow tubes and the cooling fins, which is very cost effective. It is still necessary, however, to braze or weld a pair of header or manifold tanks to the ends of the round flow tubes in order to feed the refrigerant into and out of the tubes. The header tanks are generally cylindrical tubes themselves, somewhat larger in diameter than the flow tubes, with a series of concave, cylindrical holes or slots punched inwardly along their length for the insertion of the ends of the core's flow tubes. The concave conical flare of the tube insertion slots acts as a lead in to assist the process of inserting the tube ends, and later provides a capillary action to create a good, leak free braze seam. Because of the distinctive appearance created by the concave flow tube insertion holes, such manifold tanks are often referred to as "piccolo" tubes. One obvious advantage of the one piece, cylindrical manifold is that it naturally creates a superior pressure vessel, easily able to withstand up to ten atmospheres or more of internal pressure. An example of the type of condenser just described, with high internal pressure resistance, may be seen in the co assigned European Patent Application 0 138 435 published Apr. 24, 1985 to Farry et al.
One disadvantage of a cylindrical manifold produced with concave, internally directed tube insertion slots is the inherent impediment created to the later installation of flow separation baffles along the length of the manifold. Such baffles divide up the refrigerant flow among the flow tubes into two or more, back and forth flow paths, somewhat similar to the flow that naturally occurs in a serpentine condenser. This can improve thermal performance in many instances. The difficulty arises from the fact that inwardly directed, concave tube slots locally disturb the smooth cylindrical inner profile of the tank. Therefore, the baffles, which are also round or cylindrical, must be inserted in place before the tube slots are punched in. Several distinctive manifold production methods have been proposed for round flow tubes, in part to ease the baffle insertion process, and also to increase the contact area between tube ends and manifold tank, so as to improve the braze joint. An SAE Technical Paper Series number 890225, entitled "Unique Manufacturing Method--Automotive Air Conditioning Condenser Manifolds" written by Jens S. Sorensen and Merle M. Cleeton, from 1989 provides a good overview of one basic design concept, which is to somehow provide external cylindrical stubs, with a significant length running perpendicular to the tank, rather than short, inwardly flared tube insertion slots. Such external stubs can be produced, at least in theory, without disturbing the cylindrical inner profile of the tank. Another alternative would be to use an internal mandrel to support the inside wall of the tank, with slots punched through the tank wall and into matching cutouts in the mandrel. Tube insertion slots so produced, however, would be inherently flat edged, that is, with little or no tube lead in surface at all, neither internal or external. Therefore, the only two practical alternatives are internal, flared tube slots, or external cylindrical tube insertion stubs. External, cylindrical stubs are expensive and difficult to produce, however. The SAE paper cited details a multi step extruding and machining process to produce the external stubs which is lengthy and results in a good deal of scrap. An alternative cold forming process is hinted at, without any particular details. Regardless of the process used to produce them, external, cylindrical tank stubs are, by definition, useful only with cylindrical flow tubes, and cylindrical flow tubes are clearly not the preferred design direction for future, high performance parallel flow condensers.
Future high performance automotive condenser designs will be driven by two very simple and obvious performance criteria. One is the fact that condensers are not inherently limited in performance by the refrigerant drop across the flow tubes, be they the many flow tubes of a parallel flow (or tube and fin) condenser or the single flow tube of a serpentine condenser. Rather, they are limited in the other direction, by the perpendicular cooling air flow forced across the tubes and fins by the same fan that pulls air through the engine cooling radiator. Such fans are limited in power. Second is the obvious fact that the flatter and thinner the flow tube, the less air pressure drop across the core it will cause. Elementary heat exchanger texts have, for decades, taught that a "flattened" or elliptical tube blocks less air flow than a cylindrical or "round" tube. Of course, the ultimate in "flat" tubes is a tube with a thin, rectangular cross section. The thinner it is, the less forced air flow it blocks. Therefore, high performance automotive condensers, now and in the future, will generally be parallel flow, with tubes as thin as it is possible for the tube manufacturers to successfully produce. That has been the clear design direction of tube manufacturers, especially those that make integral, extruded aluminum tubing, since the late 1960's. That is, thinner and thinner tubing, as allowed by advances in their technology, such as improved extrusion die design, higher pressure presses, and improved aluminum alloys.
There are some unique manufacturing issues with parallel flow condensers using flat tubes, as compared to tube and fin condensers using round tubes. Condensers using flat tubing typically use corrugated cooling fins brazed between the flat surfaces of each parallel pair of tubes, instead of planar, flat fins. This is because it is essentially impossible to practically mechanically expand the inside of a very thin, flat tube. In earlier designs, flat tube parallel flow condensers did not typically use cylindrical, one piece manifold tanks. Instead, they used rectangular, two piece tank designs, three sided trough shaped rear piece closed by a slotted header plate at the front. This is because, with the relatively wide flat tubes in use in the early 1980's, a cylindrical manifold tank of round cross section would have been volumetrically inefficient. It would had to have a diameter at least equal to the width of the tubes, making it far larger in volume than necessary. Now that flat tubes have become narrower as well as thinner, cylindrical, one piece manifold tanks for flat tube condensers are potentially practical. However, one piece round manifold tanks for flat flow tubes face the same problem relative to insertion of flow separators.
The typical design disclosed for a round manifold feeding flow into and out of flat condenser tubes is simply the flat tube equivalent of the punched in cylindrical tube insertion slots described above for round tubes. That is, the slots are still punched inwardly, and flared inwardly, but are basically oblong or rectangular in profile, rather than round. Again, one of the inevitable results of inwardly punching closely spaced slots through the tank walls without supporting the inner profile of the tank wall is that the round inner profile of the tank is severely deformed and disturbed. A well illustrated example of the inevitable disturbance of the round tank profile can be seen in U.S. Pat. No. 4,615,385 issued Oct. 7, 1986, to Saperstein, et al. While this creates a desirable tube lead in surface, it makes the later insertion of round flow separators impossible, at least without cutting a dedicated insertion slot through the back of the tank for the separator. A typical separator insertion slot cut through the back of a tank may be seen in U.S. Pat. No. 5,246,064 issued Sep. 21, 1993, to Hoshino et al. As a consequence, some designs attempt to obtain the pressure resistance benefits of a single piece round tank with a two piece round tank, in which two half cylinders are sandwiched together around the separators. An example may be seen in U.S. Pat. No. 5,329,995 issued Jul. 19, 1994, to Dey et al. While the separator insertion slot is eliminated in two piece designs, two fill length braze seams are added instead. As an alternative to punching the tube slots inwardly and disturbing the tube profile, U.S. Pat. No. 5,052,478 issued Oct. 1, 1991, to Nakajima et al. discloses a method of supporting the inner surface of the tank with a cylindrical arbor as the tube slots are punched out. However, with this method, the lead in surface available in the tube slot to guide the flow tube ends into place is limited in thickness to not much more than the wall thickness of the tank material itself. Interestingly, even with the internal arbor to preserve the tank's round inner profile, the use of flow separator insertion slots cut through the back of the tank is still disclosed. Apparently, just the process of punching slots through the wall of even an arbor supported tank wall creates enough burr on the slot edges to prevent the later ram rod type insertion of round flow separators.